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Hot Rods Question for engine builders

Discussion in 'The Hokey Ass Message Board' started by Rusty O'Toole, Apr 26, 2018.

  1. Rusty O'Toole
    Joined: Sep 17, 2006
    Posts: 9,578

    Rusty O'Toole

    Suppose you were building a long stroke engine from the fifties or earlier. And suppose you were using new pistons that were lighter than the originals. Would it be necessary to rebalance the engine? Would it make any difference if it was a 4, 6 or 8 cylinder, straight eight or V8? Is there any drawback to using light pistons?
  2. drtrcrV-8
    Joined: Jan 6, 2013
    Posts: 1,453


    If you're going to spend the money to build ANY engine "right", is there any valid reason NOT to spend another $250+/- that will not only help make it smoother, but "live longer" as well? I do not see any drawback to lighter pistons, & the motor will usually tend to be more responsive.
  3. Tickety Boo
    Joined: Feb 2, 2015
    Posts: 1,359

    Tickety Boo
    from Wisconsin

    How much lighter? You might be able to make up the difference with a thicker wall wrist pin.
    Deuces likes this.
  4. How much lighter are we talking here?
    Yes, you should re-balance. As a plus, you get to take weight out of the crank too.
    If you wanted to get fancy, you could cut down the counterweights some, rather than drilling a bunch of holes that create windage.

  5. CNC-Dude
    Joined: Nov 23, 2007
    Posts: 943


    If it's an inline engine, the weight of the rods and piston do not affect the balance of the engine at all going from heavier to lighter or vice versa as long as the rods and piston themself all weigh the same. In a V-style engine it will make a big difference. So can you be more specific as to whether your meaning an inline or V-style engine.
    Truck64, ottoman and Boneyard51 like this.
  6. Deuces
    Joined: Nov 3, 2009
    Posts: 20,525

    from Michigan

    I like the heavier wrist pin idea to make up the difference between the stock and aftermarket lightweight pistons....
  7. Budget36
    Joined: Nov 29, 2014
    Posts: 6,717


    Always wondered about this type of thing...I mean if the crank is in balance...the rods are in balance, and you change pistons of a different weight (assuming they are matched), why would you, me or that other guy, need to rebalance anything?

    I've assembled several engines after machine work with new pistons, never thought to have anything re-balanced. Then again I've never turned 8500 rpms.

    I'd like to see more input on this as well.

  8. If when you put it together it vibrates it needs balancing.
    Deuces likes this.
  9. ^^^^^ You make it sound easy. Just reset the wash in a washing machine ;)
    Truck64 likes this.
  10. jimmy six
    Joined: Mar 21, 2006
    Posts: 9,459

    jimmy six

    On a 6 cylinder GMC the crank is balanced with no bobweights. If you weigh your rods, pistons, pin, rings, etc and they are all within 1 gram of each other all inclusive you should not have a problem. I cannot speak for any other engine. If your going to race it or keep it....balence it.
    302GMC likes this.
  11. e z i
    Joined: Sep 5, 2007
    Posts: 578

    e z i

    As long they all weight the same, it shouldn't matter how much lighter/ heavier they are over factory pistons.
  12. Boneyard51
    Joined: Dec 10, 2017
    Posts: 5,025


    CNC, could you elaborate on why an inline 6 doesn’t need to be balanced and a V8 does? Bones
  13. Yes, please. If I tried to think thru this on my own it's just going to end up making my brain hurt. :mad: I knew this was going to raise some interesting questions and even more interesting answers. :rolleyes:
    Joined: Mar 22, 2006
    Posts: 2,059

    from OHIO


    Sent from my iPhone using The H.A.M.B. mobile app
    Boryca, Deuces and ottoman like this.
  15. big duece
    Joined: Jul 28, 2008
    Posts: 6,225

    big duece
    from kansas

    When they balance the crank they use bob weights that mimic the weight of your rod, rod bearing, piston, pin, rings, possibly a thing coating of oil. If ANY of those items are changed, they crank needs to be balanced accordingly. CDP-D.jpg
    ottoman and GOATROPER02 like this.
  16. ottoman
    Joined: May 4, 2008
    Posts: 309

    from Wisconsin

    Ah I guess you never balanced a V type motor then?
  17. MAD MIKE
    Joined: Aug 1, 2009
    Posts: 626

    from 94577

    Seems more like he's referencing to the perfect First Order and Second Order balance that Triples, Pea Shooters and 60° V12s have(120° crank throws).
    No reciprocating combustion engine I know is immune to imbalance.
  18. deucemac
    Joined: Aug 31, 2008
    Posts: 1,138


    In the immortal words of Smokey Yunick " Balancing takes the buzz out of the steering wheel at 150". I always check and rebalance as necessary whether it is a a stock rebuild or up to a Bonneville long course motor. I have seen factory balance be all over the charts.
  19. Deuces
    Joined: Nov 3, 2009
    Posts: 20,525

    from Michigan

  20. CNC-Dude
    Joined: Nov 23, 2007
    Posts: 943


    Only V-style engines are balanced using bobweights. Inline engine are not, the crank is spun and balanced by itself and the weight of the rods, pistons, wrist pins, etc do not effect the balance of the crankshaft. And those components are balanced separately.
    ottoman likes this.
  21. 4wd1936
    Joined: Mar 16, 2009
    Posts: 969

    from NY

    "big deuce" along with a few others has it right. The faster you go, the bigger difference in piston weight the more necessary it is to rebalance that crank.
  22. CNC-Dude
    Joined: Nov 23, 2007
    Posts: 943


    Many people are familiar with the methods and ways a V-style engine are balanced, and often confuse or assume and inline engine is balanced in the same way, and they are not. Inline engines aren't balanced using bobweights like "V" engines are, so there are no bobweights to determine or add to the crank to balance. On inline engines, the counterweights on the crank oppose each other like a seesaw, and the weight of the rods, pistons, wrist pins,etc.. is irrelevant to the balance of the crankshaft, as long as the total combined weight of those components are the same as the others, because they also oppose each other like a seesaw and counteract, or balance themselves.
    A "V" engine is completely different, any weight that is added to the rotating weight in the way of heavier/lighter rods, pistons, wrist pins beyond a certain amount(sometimes as little as 30 grams)from what the crank was originally balanced to will require a re-balance to compensate for the heavier/lighter components via adding bobweights that are calculated from a formula to determine the amount needed. Also, grinding or lightening a "V" crank from knife-edging or turning down counterweights or drill the rod pins will also naturally require re-balancing the crank from its original balance even if the stock rods or pistons haven't been changed. Grinding or lightening an inline crank by knife-edging or turning the counterweights down will also require just the crank itself to be re-balanced. But still, bobweights are required for this only in the "V" engine, but not in an inline engine. These are two completely different animals in respect to how they are balanced and how heavier/lighter rotating assemblies affects them differently.
    Last edited: Apr 27, 2018
    ottoman, harpo1313 and henryj1951 like this.
  23. bobw
    Joined: Mar 24, 2006
    Posts: 2,368


    I vote for CNC-Dude's approach and so does the 300 Ford 6 in my dragster that was balanced his way.
  24. buffaloracer
    Joined: Aug 22, 2004
    Posts: 805

    from kansas

    I balance most of the engines I put together. If it's an inline engine I make the parts all weigh like each other and check the crank on a couple of knife edges. I've made up bob weights for v6s and v8s and have a friend who cuts me a break on balancing the crank. After balancing the crank we install the harmonic dampener and flex plate or fly wheel and do it again taking weight off of the dampener or plate.

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